This invention relates to a method of control for an analog display device of the non-continuous liquid crystal strip type and to a circuit for the practical application of said method. The invention finds an application in the field of electro-optics and especially in the display of physical quantities such as voltage, temperature, velocity, time and so forth.
A known type of liquid crystal analog display device is constituted by a strip of liquid crystal film interposed between two sets of electrodes. Under excitation, a device of this type can assume, for example, a first optical state on one portion of the strip and a second state on the remainder of the strip. The length of one of these portions constitutes an analog representation of a physical quantity.
In some cases these display devices have a continuous character since the electrodes chosen are continuous strips. One known device which can be mentioned by way of example and falls in this class consists of a conductive electrode and a resistive electrode placed in oppositely-facing relation. A device of this type is described in particular in the article by R. A. Soref, entitled "Electronically Scanned Analog Liquid Crystal Displays" and published in the June, 1970 issue of "Applied Optics", vol. 9, No 6, page 1323.
In other devices, the electrodes have a noncontinuous structure. These devices are constituted by a plurality of juxtaposed electrodes of small size. The display operation is again performed by means of the length of a strip but is subject in this case to non-continuous variation. A device of this type is described, for example, in the article by W. L. Carl and C. R. Stein entitled "A Nonscanning Matrix Addressing Scheme for Certain Liquid Crystal Displays", published in the reports on the symposium of the Society for Information Display (SID) held in 1976 at Beverley Hills, Chapter 4.4, page 40, and in the article by S. Sherr entitled "A Liquid Crystal Bar Graph Meter" and published in the same reports, Chapter 4.5, page 42.
The control of liquid crystal display devices formed by a plurality of excitation points gives rise to particular problems. It would of course be possible to control each point of the liquid crystal strip separately. This method would lead to a very high degree of complexity of the connection system since the number of connections would have to be equivalent to the number of elements to be controlled.
For the reason just stated, it is usually found preferable to group the different points together in sectors. Control of the display at one point is accordingly split up into control of a sector and control of a point of a predetermined order. If the entire strip of a display system consists of N points, said strip is therefore split up into S sectors of P points with S.P.=N. In practice, this consists in providing electrodes in the form of S plate-type electrodes, there being placed opposite to these latter P point-type electrodes. According to its order, each point electrode is joined to one particular connection of a group of P point connections and each plate electrode is joined to one particular connection of a group of S plate connections. The total number of connections is thus reduced from S.times.P to S+P.
Control of the analog display of a physical quantity by means of a device of this type is carried out by applying a first set of voltages to the S plate connections and a second set of voltages to the P point connections. This particular structure and the resultant method of control recall the matrix structures of certain crossbar display devices in which the display is carried out by applying voltages to the columns and to the lines of the matrix.
As in the case of the matrix structure, control of the single-strip device can be sequential if excitation signals are applied sequentially to the plate electrodes or else it can be non-sequential if the control voltages are applied simultaneously to all the plates.
However, analog display along a strip has specific characters which are not found in matrix devices. In point of fact, the display of a strip of variable length makes it necessary for the different sectors of the strip to be of two types: in the case of all sectors except one, the displayed points all have the same optical state (either 1 or 0); in the case of only one of the sectors the points have in some cases the state 1 and in other cases the state 0. This special sector will be designated hereafter as a boundary sector and determines the position of the zone of transition between that portion of the strip which is in state 1 and the remainder of the strip which is in state 0. A logical system of this type is not usually present in matrix devices since the display of a matrix in which all the strips except one are either in state 1 or in state 0 would be of no practical interest.
The methods of control for strip-type analog display devices therefore have distinctive characters. In the first place, they must ensure that the voltages applied to the point electrodes and to the plate electrode of the boundary electrode correctly initiate the appearance of the two possible states in this boundary sector. In the second place, they must ensure that the voltages which inevitably appear on the other point electrodes of the other sectors (by reason of the interconnection of the point electrodes) result in correct display on the other sectors, that is, a uniform display either in state 1 or in state 0.
In order to solve this problem, it has been proposed, especially in the aforementioned article by W. L. Carl and C. R. Stein, to carry out the discrimination between displayed points and non-displayed points by means of the excitation frequency. In more exact terms, the following procedure is adopted for controlling the boundary sector. An a.c. voltage having a frequency f.sub.0 is applied to the plate electrode whilst an a.c. voltage having the same frequency f.sub.0 and the same amplitude as the preceding is applied to the point electrodes of said boundary sector; the last-mentioned voltage has the same phase if it is desired to display the state 0 and is in opposite phase it it is desired to display the state 1.
A voltage having a frequency which is higher than the cut-off frequency of the liquid crystal is applied to the plate electrodes located on one side of the boundary sector in which it is desired to obtain a uniform appearance corresponding to the state 0. By way of example, said frequency has a value of 20 f.sub.0. The voltages applied to the point electrodes of these sectors remain the same as those applied to the point electrodes of the boundary sector since the point electrodes are interconnected. The high-frequency voltage applied to the flat electrode imposes an optical state 0 on the liquid crystal irrespective of the phase of the voltage applied to the point electrodes. The contrast obtained in this zone which is located on one side of the boundary sector is accordingly uniform.
Control of the sectors located on the other side of the boundary sector is carried out by applying to the plate electrodes of these sectors a voltage having a frequency 2 f.sub.0 and an amplitude equal to 1.73 times the amplitude applied to the point electrodes. In consequence and irrespective of the phase of the voltages applied to the point electrodes, all the points of said sectors are excited in the same manner and are in the optical state 1.
This method of control is therefore of considerable complexity since it entails the use of signals having three different frequencies: f.sub.0, 2 f.sub.0 and 20 f.sub.0. Moreover, the method presupposes the existence of a cut-off frequency at which the dielectric anisotropy of the liquid crystal undergoes a change of sign and is therefore not applicable to all liquid crystals.